Germanium enriched silicon material for making solar cells

ABSTRACT

Techniques for the formation of silicon ingots and crystals using silicon feedstock of various grades are described. A common feature is adding a predetermined amount of germanium to the melt and performing a crystallization to incorporate germanium into the silicon lattice of respective crystalline silicon materials. Such incorporated germanium results in improvements of respective silicon material characteristics, including increased material strength and improved electrical properties. This leads to positive effects at applying such materials in solar cell manufacturing and at making modules from those solar cells.

CLAIM OF PRIORITY

This application is a Continuation-In-Part of and claims the benefit ofpriority under 35 U.S.C. §120 to U.S. Patent Application SerialNo.12/140,104, filed Jun. 16, 2008. This application also claims thebenefit of priority to PCTUS2009/047539, filed Jun. 16, 2009 nowpublished as WO 2010/005736 on Jan. 14, 2010. Each document is alsoincorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to methods and systems for use in thefabrication of semiconductor materials such as silicon, for exampleusing lower-grade silicon. More particularly, the present disclosurerelates to a method and system for forming a silicon crystal or ingotwith improved mechanical and electrical characteristics, using feedstockmaterial of various grades and germanium enrichment.

BACKGROUND

The photovoltaic (PV) industry is growing rapidly and is responsible foran increasing amount of silicon being consumed beyond the moretraditional uses as integrated circuit (IC) applications. Today, thesilicon needs of the solar cell industry are partially competing withthe silicon needs of the IC industry. With present manufacturingtechnologies, both IC and PV industries require a refined, purified,silicon feedstock as a raw silicon starting material.

Materials alternatives for the bulk of current solar cells range frommono-crystalline silicon wafers, for example based on very clean rawsilicon such as electronic-grade (EG) silicon feedstock needed for theIC industry, to multi-crystalline (mc) silicon wafers based on not asclean raw silicon such as the so-called called solar-grade (SOG) siliconfeedstock or an even lower-quality material called upgradedmetallurgical-grade (UMG) silicon feedstock.

Low-grade feedstock materials for the PV industry, such as UMG silicon,are typically processed into ingots and wafers of mc-silicon where theultimate, solar cell relevant quality is typically controlled by grainboundaries, other structural defects and a relatively high concentrationof impurities such as transition metals. Also carbon related and oxygenrelated defects in the wafer bulk can degrade cell properties, inparticular when associated with metals. Some species of the broad defectspectrum may be passivated with hydrogen to reduce their electricaldegradation potential.

Higher-grade feedstock materials for the solar cell industry, such as EGsilicon, are typically processed into mono-crystals and, subsequently,wafers with mono-crystalline structure where the ultimate, solar cellrelevant quality is controlled by impurities similar to the case ofmc-silicon described above. There are two well-established growingtechniques for mono-crystals of silicon (in the following calledcrystals). By far dominant is the Czochralski (CZ) technique where a CZcrystal is pulled out of a silicon melt residing in a quartz crucible.Medium to high-grade feedstock silicon is employed for generating the CZsilicon melt. A more sophisticated alternative is the Floating Zone (FZ)technique where a FZ crystal is grown by “floating” a small melt zonethrough a so-called supply rod of high-grade feedstock silicon. One wayof getting predetermined amounts of elements into FZ crystals isso-called “pill doping” into supply rods before generating the meltzone. Typically, FZ silicon crystals contain less impurities than CZcrystals, mainly because no crucible is required.

In any case, since silicon is brittle at room temperature, there is thegeneral problem of wafer breakage at wafer and solar cell processing andhandling, including the manufacturing of modules out of solar cells.Consequently, the mechanical strength of silicon wafers and relatedsolar cells is also an important quality factor in the PV industry,besides electrical properties. This holds for mono-crystalline materialand equally for multi-crystalline ingot material.

Wafer breakage is initiated by crack formation and subsequentpropagation. Cracks may originate from, for example, handling-inducedlocal damage on surfaces, in particular at edges and corners.State-of-the-art solar cell manufacturing technology uses carefulhandling and processing of wafers and solar cells to avoid suchsituations. Intrinsic material strength of bulk silicon is also afunction of bulk lattice defects. Of particular concern are defects thatgenerate local tensile lattice strain, enabling internal crackformation/propagation at reduced external force (relative to an ideallattice structure).

A need exists for a simple process that delivers UMG-basedmulti-crystalline silicon material with good ingot yield and improvedmechanical and electrical properties, the latter in regard to solar cellquality. Such a process should be easily transferable to higher-grade,non-UMG feedstock silicon which is used partially or exclusively forproducing mono-crystalline silicon materials, for example by applyingthe CZ technique or the FZ technique.

SUMMARY

A technique is here disclosed for the crystallization of silicon whichmay be useful for ultimately making solar cells. The present disclosureincludes a method and system for making silicon ingots or crystals withimproved electrical and mechanical material characteristics, for use ina variety of solar cell applications.

The resulting solar cells may be shipped, installed, and used withoutconcern for strong susceptibility to breakage. In addition to deliveringimproved mechanical strength, improved electrical properties of thesilicon material resulting from related ingots or crystals may also leadto higher ingot/crystal yield, measured as ingot/crystal portion with arecombination lifetime of certain minimum level needed to reach criticalcell efficiencies.

According to one aspect of the disclosed subject matter, a silicon ingotforming method and associated system are provided for using a low-gradesilicon feedstock that includes forming within a crucible device amolten solution from a low-grade silicon feedstock and a predeterminedamount of germanium. The process and system perform a directionalsolidification of the molten solution to form a silicon ingot within thecrucible

According to another aspect of the disclosed subject matter, a siliconingot forming method and associated system are provided for adding notonly a predetermined amount of germanium but also a predetermined amountof Ga to a silicon feedstock of various grades. According to anotheraspect of the disclosed subject matter, a silicon crystal forming methodand associated system are provided for using a higher-grade siliconfeedstock and a predetermined amount of germanium. The process andsystem perform a crystallization of the molten solution to form asilicon crystal.

In one case, crystallization is achieved using the CZ technique, where apredetermined amount of germanium is added to the higher-grade siliconfeedstock before meltdown and subsequent CZ crystal pulling.

In another case, crystallization is achieved using the FZ technique,where the predetermined amount of germanium is attached to thehigh-grade silicon supply rod before applying the floating melt zone forFZ crystal growing.

The predetermined amount of germanium can be added in pure form. It canbe also part of a compound such as a pure silicon-germanium alloy.

These and other advantages of the disclosed subject matter, as well asadditional novel features, will be apparent from the descriptionprovided herein. The intent of this summary is not to be a comprehensivedescription of the claimed subject matter, but rather to provide a shortoverview of some of the subject matter's functionality. Other systems,methods, features and advantages here provided will become apparent toone with skill in the art upon examination of the following Figures anddetailed description. It is intended that all such additional systems,methods, features and advantages be included within this description, bewithin the scope of the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the disclosed subject matter maybecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which reference charactersidentify correspondingly throughout and wherein:

FIG. 1 is a prior art general process for the formation of a solar cellbeginning with the formation of a silicon ingot;

FIG. 2 illustrates conceptually a process flow for producing a siliconingot with improved characteristics according to the present disclosure;

FIG. 3 provides a process flow for one embodiment the present disclosureemploying low-grade raw silicon feedstock for a directionalsolidification ingot formation process;

FIG. 4 provides a process flow for one embodiment of the presentdisclosure employing higher-grade raw silicon feedstock for a CZ crystalpulling process;

FIG. 5 provides a process flow for one embodiment the present disclosureemploying a supply rod of high-grade raw silicon feedstock for an FZcrystal growing process;

FIG. 6 illustrates conceptually another process flow for producing asilicon ingot with improved characteristics according to the presentdisclosure;

FIG. 7 shows a diagram of breakage test results on a plurality ofdifferent wafers tested from a reference ingot and an engineered ingotusing an embodiment of the present disclosure;

FIG. 8 illustrates a diagram of breakdown voltage test results on aplurality of different wafers tested from a reference ingot and anengineered ingot using an embodiment of the present disclosure.

DETAILED DESCRIPTION

The method and system of the present disclosure provide a semiconductoringot formation process for producing a silicon ingot or crystal using alow purity or high purity silicon feedstock. As a result of using thepresently disclosed subject matter, an improvement in the properties ofingots formed from low-grade semiconductor materials, such as upgradedmetallurgical grade silicon (UMG) occurs. Such improvement allows use ofUMG silicon, for example, in producing solar cells for applications insolar power generation. The method and system of the present disclosure,moreover, particularly benefits the formation of silicon-based solarcells using UMG or other non-electronic grade feedstock materials. Thepresent disclosure may, therefore, allow the manufacture of solar cellsin greater quantities and in a greater number of fabrication facilitiesthan has heretofore been possible.

Laying a context for the present disclosure, FIG. 1 depicts a knownprocess 10 beginning at step 12. At step 12, MG or other low-gradesilicon enters known wafer forming process flow 10. Known process flow10 extracts high-grade silicon from MG silicon at step 14. High-gradesilicon extraction step 14 is a high-cost processing sequence resultingin EG silicon or somewhat relaxed silicon quality called SOG feedstockquality. Those are the types of silicon feedstock materials used formaking the ingot in step 16. Known process flow 10 includes slicing thesilicon ingot, generally using a wire-saw to derive a silicon wafer atstep 18. The resulting silicon wafers then enter solar cell formationprocess 20 using the resulting wafer.

FIG. 2 depicts, in general terms, novel aspects of how the disclosedprocess may be integrated into the overall solar cell fabrication flow30. The improved silicon ingot characteristics arising from the presentdisclosure may include greater mechanical strength , and betterelectrical characteristics such as recombination lifetime of theresulting wafers, and hence the resulting solar cells, using feedstockmaterial of various grades and the presently disclosed germaniumenrichment steps.

Fabrication flow 30 includes using MG silicon at step 32 that may bepurified to some degree to become UMG silicon. The resulting siliconquality still results in low-grade silicon 34. Accordingly, siliconquality 34 relates to much lower cost as compared to silicon quality 14.Also, low-grade silicon ingot 34 includes a higher content of metallicand non metallic impurities as compared to silicon quality 14. Thepresent disclosure includes the addition or enhancement of apredetermined quality and quantity of germanium 36 for the purpose ofimproving the mechanical strength, and electrical properties of theresulting ingot material. The combination of silicon and germanium areheated to form a silicon melt as an initial aspect of ingot formationstep 38.

At step 38, silicon ingot formation may occur using, for example, adirectional solidification process, a CZ crystal formation process, or aFZ crystal formation process. Adjustment of the crystallizationconditions based on actually applied germanium concentrations furtherenhances mechanical properties and electrical properties. Step 40represents the formation of silicon wafers. Finally, the solar cellforming process occurs at step 42.

FIG. 3 provides a process flow 50 for one embodiment of the presentdisclosure employing low-grade raw silicon feedstock. In process flow50, a first step 52 includes placing low-grade raw silicon (e.g., UMGsilicon) into a crucible. Before the heating process for silicon meltformation begins, the present disclosure contemplates the addition, atstep 54, of a predetermined amount of pure germanium (e.g., germaniumwith a purity of 99.99 percent or 99.999 percent) to the low-gradesilicon feedstock.

The total range of added germanium in improved silicon may range from 5to 200 ppmw. Another embodiment may allow for a range of germanium from5 to 50 ppmw. Another embodiment may allow for a range of germanium from20 to 40 ppmw. Another embodiment may allow for a range of germaniumfrom 30 to 40 ppmw. Another embodiment may allow for a range ofgermanium from 50 to 100 ppmw. Another embodiment may allow for a rangeof germanium from 50 to 200 ppmw. Another embodiment may allow for arange of germanium from 100 to 150 ppmw. Another embodiment may allowfor a range of germanium from 120 to 180 ppmw.

Once the combination of solid low-grade silicon and pure germaniumreside in the crucible, step 56 includes heating the solid mixture forgenerating a melt of the low-grade silicon and added germanium at step58. The molten low-grade silicon and germanium may be subsequentlycrystallized, step 60, by performing a directional solidification, forexample.

FIG. 4 provides a process flow 70 for a further embodiment of thepresent disclosure employing higher-grade raw silicon feedstock. Inprocess flow 70, a first step 72 includes placing higher-grade rawsilicon (e.g., EG silicon) into a crucible. Before the heating processfor silicon melt formation begins, the present disclosure contemplatesthe addition, at step 74 of a predetermined amount of pure germanium(e.g., germanium with a purity at least of 99.999) to the higher-gradesilicon feedstock.

Once the combination of solid low-grade silicon and pure germaniumreside in the crucible, step 76 includes heating the solid mixture forgenerating a melt of the higher-grade silicon and added germanium, step78. A portion of molten higher-grade silicon and germanium may besubsequently formed into a silicon crystal, step 80, by pulling a CZcrystal using established procedures for achieving and maintainingdesired crystal properties throughout the CZ process.

FIG. 5 provides a process flow 90 for a further embodiment the presentdisclosure starting with a supply rod of high-grade raw silicon,specifically EG silicon feedstock. In process flow 90, a first step 92includes beginning with a high-grade raw silicon (e.g., EG silicon)supply rod. The supply rod allows the use of a floating zone or FZregion for an FZ crystallization process. In association with formingthe FZ region, the present disclosure contemplates the addition, at step94, of a predetermined amount of pure germanium (e.g., germanium with apurity at least of 99.999) to the supply rod of high-grade raw siliconfeedstock.

Once the combination of solid high-grade silicon and pure germaniumexist in the FZ, step 96 includes using the floating melt zone of thehigher-grade silicon and added germanium, step 98, to subsequently forma silicon crystal, step 100, by growing an FZ crystal from the supplyrod and germanium mixture. At this point, the established procedures forachieving and maintaining desired crystal properties throughout the FZprocess may find application.

FIG. 6 depicts, in general terms, novel aspects of another process thatmay be integrated into the overall solar cell fabrication flow 130.Fabrication flow 130 includes using MG silicon at step 132 that may bepurified to some degree to become UMG silicon. The resulting siliconquality still results in low-grade silicon 134. Accordingly, siliconquality 134 relates to much lower cost as compared to unmodified siliconas described in FIG. 1. Also, low-grade silicon ingot 134 includes ahigher content of metallic and non metallic impurities as compared tounmodified silicon. The present disclosure includes adding germanium inconjunction with further addition or enhancement of a predeterminedquality and quantity of gallium 136 for the purpose of further improvingproperties, including mechanical and electrical properties of theresulting ingot. In one example gallium is added at concentrations in arange of 0 to 10 ppmw. The combination of silicon with germanium andgallium are heated to form a silicon melt as an initial aspect of ingotformation step 138.

FIG. 7 depicts a characteristic result 110 of an experiment comparingthe mechanical wafer strength of an example of germanium-doped materialfrom ingot B with non-doped reference material from ingot A. For bothingots the very same type of UMG feedstock silicon has been selected,and the casting was done sequentially with the same tool applying thesame casting conditions. Then, from each ingot one set of near-bottomwafers (116 and 120) and one set of near-top wafers (118 and 122) wereselected for determining the mechanical wafer strength, measured asratio of maximum external force F_(max) over maximum wafer deformationI_(max) in a standard 4-line bending test. The normalized wafer strength(strength divided by wafer thickness) 112 is presented for the variouswafer groups where the sequence number 114 describes the originallocation within respective ingots (increasing numbers from bottom totop). From the graph we see that wafers from the germanium-doped ingot Bexhibit higher strength than wafers from the reference ingot A. Theresults shown here support the conclusion that the addition of germaniumin the formation of a silicon ingot yield greater strengthcharacteristics than a silicon ingot made from an otherwise identicallyformed silicon ingot.

These results are further substantiated by the Table below, which showsan example of additional improvements achieved with material from agermanium-doped silicon ingot.

TABLE INGOT YIELD, CARRIER LIFETIME, AND EFFICIENCY INCREASE Increase ofIncrease of Increase in ingot yield recombination lifetime efficiency44.7% 20.7% 1.2%

The Table reports data for a medium-grade feedstock that was used information of a multi-crystalline silicon ingot. The above data showsimprovements of electrical material characteristics, on top of improvedmechanical characteristics described above. In the Table, measuredimprovements of such electrical characteristics are given as percentageof increase in enhanced silicon, as described in embodiments above, ascompared to silicon that has not been modified consistent with thepresent disclosure. Because of the improved material properties, thereresults a corresponding yield increase in possible number of siliconwafers and related solar cells from such an ingot. Moreover, improvedelectrical characteristics translate into an average yield increase forthe resulting solar cells.

In other words, the improved material characteristics of a silicon ingotformed consistent with the teachings of the present disclosure have acascading effect to promote a corresponding reduction in the final costsassociated with the manufacture of solar cells and systems using suchsolar cells. That is, because the germanium doped silicon materialexhibits improved material strength and flexibility over non-dopedsilicon material, a greater likelihood exists that the mechanicalprocesses of slicing the wafers from the ingot will result in less waferbreakage. Then, once the wafers are sliced, the continuing materialstrength and flexibility of the silicon wafers provide increaseddurability as such silicon wafers are further formed into solar cells.Moreover, such resulting solar cells are less likely to break, crack, ordemonstrate fracture stress upon installation or shipping from the solarcell manufacturing site to the points of assembly as solar cell arraysand the final installation in the field of such solar cell arrays.Lastly, the increased durability and flexibility of such solar cells mayfurther increase the operational life of the solar cell arrays, asweathering, thermal and environmental transients in the field may damageor otherwise occur.

From the increased yield of silicon wafers, the increased yield of solarcells, the increased yield of solar cell arrays, and the increasedmechanical durability of the solar cell arrays embodying the teachingsof the present disclosure, highly significant economies may arise in thesolar cell industry. Such economies directly and materially translate toreduced costs in the generation of electricity from solar cells.

As the above Table depicts, the germanium-doping example may cause anincrease in not only the carrier lifetime of respective silicon materialbut also the overall solar cell efficiency. This is seen, for example,in the demonstrated increase in recombination lifetime of 20.7% and ameasured cell efficiency increase of 1.2% in the above Table.

FIG. 8 further illustrates further improvements in electrical propertiesof the silicon material as a result of an addition of germanium asdescribed in the present disclosure. The Figure shows breakdown voltageversus a wafer identification number, where a lower wafer identificationnumber indicates a location near a bottom of an ingot, and a higherwafer identification number indicates a location near a top of an ingot.As can be seen from the Figure, the reference ingot data 150 showsconsistently lower breakdown voltages than ingots processed according toembodiments of the invention 152.

In one example, selecting an effective amount of germanium for additionto the silicon depends on a number of potentially competing factors. Forexample although mechanical properties such as strength and flexibilitycan be enhanced by adding more germanium, additions over a certainconcentration can introduce unwanted effects such as formation ofsilicon carbides. Therefore, for example, concentrations of germanium inthe range of 5 to 50 ppmw provide enhanced electrical properties such aslow light-induced degradation and high recombination lifetimes, alongwith enhanced mechanical strength and low dislocation densities, whilekeeping unwanted effects to a minimum. In one example, an effectiveamount of germanium includes concentrations of germanium in the range of0 to 20 ppmw. In one example, an effective amount of germanium includesconcentrations of germanium in the range of 30 to 60 ppmw.

The silicon material improvements of the present disclosure may derivefrom increased compressive lattice strain associated withsubstitutionally incorporating germanium atoms in the lattice structureof crystalline silicon. Such substitutional incorporation of germaniummay compensate local tensile stresses associated with certain bulkdefects in silicon wafers or solar cells and result in improved controlof the intrinsic material strength.

Empirical results indicate that a silicon material with germanium insufficient amounts demonstrates an increased material strength. The bestpractical range depends on the material quality generated. Slightlyhigher germanium concentrations turn out to work better formono-crystalline silicon, as compared to multi-crystalline silicon.

In summary, the disclosed subject matter provides a method and systemfor forming a silicon ingot or crystal which includes forming within acrucible device a molten solution from silicon feedstock and apredetermined amount of germanium, followed by either directionalsolidification to form an ingot within the crucible, pulling CZ crystalsfrom the melt, or growing FZ crystals.

Although various embodiments that incorporate the teachings of thepresent disclosure have been shown and described in detail herein, thoseskilled in the art may readily devise many other varied embodiments thatstill incorporate these teachings. The foregoing description of thepreferred embodiments, therefore, is provided to enable any personskilled in the art to make or use the claimed subject matter. Variousmodifications to these embodiments will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other embodiments without the use of the innovative faculty.Thus, the claimed subject matter is not intended to be limited to theembodiments shown herein, but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

What is claimed is:
 1. A method for forming crystalline silicon havingimproved mechanical and electrical characteristics, comprising the stepsof: initiating a silicon crystallization process using a predeterminedamount of a silicon feedstock material; adding to said silicon feedstockmaterial a predetermined quantity of germanium, wherein the quantity ofgermanium ranges from 5 to 50 ppmw; generating a melt from respectivesilicon feedstock material and said quantity of germanium; andperforming a crystallization of said melt.
 2. The method of claim 1,further comprising the step of initiating a directional solidificationsilicon crystallization process using a predetermined amount of siliconfeedstock material.
 3. The method of claim 1, further comprising thestep of initiating a directional solidification silicon crystallizationprocess using a predetermined amount of UMG silicon feedstock material.4. The method of claim 1, further comprising the step of initiating a CZsilicon crystal pulling process using an EG silicon feedstock material.5. The method of claim 1, further comprising the step of initiating a CZsilicon crystal pulling process using an SOG silicon feedstock material.6. The method of claim 1, further comprising the step of initiating anFZ silicon crystallization process using an EG silicon supply rod. 7.The method of claim 1, wherein adding to said silicon feedstock materialthe quantity of germanium includes adding germanium with a minimumpurity level of 99.999 percent purity.
 8. The method of claim 1, whereinadding to said silicon feedstock material the quantity of germaniumincludes adding germanium in a silicon-germanium alloy of the formSi_(x)Ge_((1-x)) with 0<x<1.
 9. The method of claim 1, wherein adding tosaid silicon feedstock material the quantity of germanium includesadding germanium in a concentration ranging from 10 to 40 ppmw.
 10. Themethod of claim 1, further comprising the step of adding to said siliconfeedstock material the quantity of germanium ranging from 30 to 40 ppmw.11. The method of claim 1, further comprising the step of adding to saidsilicon feedstock material a combination of germanium and gallium. 12.The method of claim 11, wherein adding to said silicon feedstockmaterial a combination of germanium and gallium includes adding galliumin a concentration between 0 and 10 ppmw gallium.